Highly integrated power management applications often require the ability to measure voltage quantities that exceed the supply voltage in magnitude. This is primarily due to a basic need to maximize efficiency by running the power management IC on as low a supply voltage as possible, while still maintaining the ability to sample and measure quantities from the surroundings that could well exceed the battery voltage.
The problem can be defined as follows: Assume that there is a low power N bit SAR ADC working from a supply voltage Vdd and with a reference voltage equal to Vref. Vref is usually equal to or slightly less than Vdd to maximize the input signal range of the ADC. The objective is to create an N+1 bit ADC which is capable of converting an input signal range from 0V to 2Vref. If 2Vref happens to be greater than Vdd then this would present a problem. First of all, a reference voltage equal to 2 Vref has to be generated from Vdd. This would mean that a power-hungry charge pump would have to be built to create a high enough voltage from which this new reference voltage can be derived. Furthermore, the charge pump would have to bias all the complementary switches (transmission gates) to eliminate the forward biasing of any body diodes in the transmission gate switches. Building a charge pump also increases the noise on this desired 2Vref reference, and an extra pin might be required for the charge pump's storage capacitor. Furthermore, the charge pump approach does not increase the effective number of bits (ENOB) by an additional bit and doubling the input signal range does not buy you an increase in ENOB, an often-desired thing when the input signal is increased. Other solutions that might involve resistor based voltage division to divide the input signal down to the 0 to Vref range would mean loading the input and possibly slowing down the conversion rate for resistor values that are high. This attenuation mechanism would render difficult an increase in the dynamic range, since the input signal gets divided down by the attenuation factor.